Synchronous detection is one technique for recovering the information signal from certain modulated carrier signals. Any linear modulation scheme (AM, DSB, SSB, and VSB) or biphase-shift keyed modulation can be demodulated by synchronous detection. Because the local oscillator of the synchronous detector must be phase synchronized (i.e., phase coherent) with the modulated carrier signal, a feedback loop, such as a phase-locked loop or a Costas loop, controls the phase of the local oscillator. The feedback loop synchronizes the phase of the local oscillator and the modulated carrier signal to ensure proper demodulation. Any synchronous detector can be tuned to provide demodulation for a range of modulated carrier signal frequencies by changing the frequency of the local oscillator. This is accomplished by opening the feedback loop, tuning the resonant circuit components of the local oscillator to the new frequency, and then closing the feedback loop. When closed again, the feedback loop will synchronize the phase of the local oscillator and the modulated carrier signal at the new frequency.
In a typical receiver employing synchronous detection, the input modulated carrier signal is band-pass filtered and down-converted to an intermediate frequency using conventional techniques. Various intermediate-frequency amplifying and filtering stages follow the down-conversion; these stages have a narrow bandwidth to provide adjacent channel rejection. Demodulation is accomplished by mixing the intermediate frequency signal with a local oscillator signal, from a voltage-controlled oscillator. The frequency of the local oscillator signal is the intermediate frequency and the phase thereof is controlled by a feedback loop so the local oscillator signal and the intermediate frequency demodulate, and the local oscillator phase is locked to the phase of the incoming modulated carrier. The frequency and phase coherent local oscillator and modulated carrier signals are mixed, producing a difference-frequency component that reduces to zero because the mixed signals are frequency and phase coherent, and a double-frequency component that is removed by low-pass filtering. The remaining component is the information signal. Because intermediate frequency components are absent in such a system, means must be included to provide adjacent channel rejection. Tuning of the local oscillator is accomplished in U.S. Pat. No. 4,408,351 by using an adder to sum an externally-produced tuning signal and the feedback signal produced by the phase-locked loop. The sum is input to the voltage-controlled oscillator and the input bandpass filter for tuning these elements to receive the desired modulated carrier signal.
As used in a typical prior art receiver, the Costas loop includes a voltage-controlled oscillator with an oscillation frequency equal to the intermediate frequency. In the Costas loop, the oscillator signal mixes, in a first mixer, with the intermediate frequency signal. The oscillator signal is also phase shifted by ninety degrees and the resulting quadrature signal is mixed, in a second mixer, with the intermediate frequency signal. The resultant signals from the first and second mixers are multiplied to produce an error signal. The error signal controls the phase of the voltage-controlled oscillator signal to match the phase of the incoming intermediate frequency signal so that demodulation thereof occurs in the first mixer. One disadvantage of the Costas loop is that it may lock onto a signal at a frequency adjacent to the desired frequency. This disadvantage can be overcome by using a crystal as the voltage-controlled oscillator. This eliminates problems associated with the Costas loop locking onto a signal at a frequency adjacent to the desired frequency, but then the local oscillator frequency can be changed only by changing the crystal. When used with an intermediate frequency technique, the Costas loop would likely use a crystal as the voltage-controlled oscillator, with the crystal frequency equal to the intermediate frequency. The intermediate frequency stages provide adjacent channel rejection, and the crystal oscillator ensures that the Costas loop locks to the correct frequency.
As an alternative to the use of the crystal as the voltage-controlled oscillator, it is possible to employ a tunable band-pass filter before the Costas loop and allow the voltage-controlled oscillator of the Costas loop to be tunable. If the band-pass filter has a very high Q only a very narrow band of frequencies will pass therethrough. With only a narrow band of frequencies presented to the Costas loop, there would be an increasingly high probability that the Costas loop would lock to the correct frequency. Such a high-Q bandpass filter would, however, be expensive and complex. This high-Q bandpass filter also provides protection against adjacent channel interference.
One attempted technique for locking the Costas loop to the correct frequency and phase is to employ both a synthesizer phase detector and a Costas detector. The synthesizer phase detector provides frequency coherency (i.e., synchronization) using an externally generated reference signal. The Costas detector provides phase coherency. The synthesizer phase detector and the Costas detector are alternately switched into and out of a feedback loop controlling the voltage-controlled oscillator, such that at one instant the Costas detector provides the error signal to the voltage-controlled oscillator and during the next instant the synthesizer phase detector provides the error signal. One disadvantage associated with this scheme is a frequency offset caused by the closing of the switch. The switch closure introduces a step voltage that shifts the frequency of the voltage-controlled oscillator. Also, the voltage-controlled oscillator can still lock onto a false or adjacent signal while the Costas detector is in control.